1. Field of the Invention
The present invention relates to electronics and, more specifically, to voltage-to-current (V/I) converters having an exponential transfer function.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
An exponential (or dB-linear) voltage-to-current (V/I) converter is a key component for the design of automatic gain-control (AGC) circuits, which are used in a variety of applications, such as wireless communications devices, hearing aids, and disk drives. A representative AGC circuit employs an exponential V/I converter in the feedback loop that controls the gain of a variable-gain amplifier (VGA). The basic form of an AGC loop, as shown in FIG. 6, consists of an alternating current (a. c.) coupled input 1 followed by a variable gain amplifier (VGA) 3 which drives a low pass filter 5 followed by an a. c. coupled output 7 to subsequent circuitry, with a feedback loop from the output 7 of the low pass filter through a peak detector 9 to an exponential voltage-to-current converter 11. Converter 11 provides as output a control signal that controls the gain of the VGA 3. The exponential characteristic of the V/I converter enables the AGC circuit to advantageously have a substantially constant settling time for a variety of initial input-signal conditions, which is very desirable for the above-specified applications. Additional details on the use of exponential V/I converters in AGC circuits can be found, e.g., in U.S. Pat. No. 6,369,618, which is incorporated herein by reference in its entirety.
One problem with exponential V/I converters is that they are not straightforwardly amenable to implementation in CMOS technology. More specifically, unlike bipolar transistors, which have an inherent exponential transfer characteristic, MOSFET transistors have a square-law transfer characteristic in strong inversion. As a result, designing a CMOS V/I converter that exhibits an exponential transfer characteristic and has other desirable properties is relatively difficult.